Electro-acoustic high impedance transducer



United States Patent Inventor Stephen L. Heidrich 29 Richmond Drive,Darien, Connecticut 06820 Appl. No. 738,192

Filed June 19, 1968 Patented Oct. 20, 1970 ELECTRO-ACOUSTIC HIGHIMPEDANCE TRANSDUCER 18 Claims, 2 Drawing Figs.

us. or 181/31 1m. Cl U GlOk 13/00 FieldofSearch 18l/31.1, 79/1155 [56]References Cited UNITED STATES PATENTS 1,837,755 12/1931 Carlisle etal.181/31 3,317,000 5/1967 Heidrich 181/31 3,324,966 6/1967 Heidrich 181/31Primary Examiner-Stephen J. Tomsky Atlorney-Ernest G. Montague PatentedOct. 20, 1970 ATTORNEY.

1 ELECTRO-ACOUSTIC HIGH IMPEDANCE TRANSDUCER The present inventionrelates to sound translating devices, in general, and toelectro-acoustic high impedance transducers which consist of an operableacoustic diaphragm with operating means in a housing enclosing air or afluid containing suitable acoustic properties wherein the enclosed fluidprovides an acoustical load to the operations of the diaphragm whichopposes the applied power variations more or less as determined by theacoustical properties of the enclosed fluid, in particular.

Loud speaker enclosures commonly used are basically of two types: thecompletely enclosed type known as the infinite baffle enclosure, andvented type known as the reflex enclosure. The present invention relatesto the infinite baffle or completely enclosed type. A characteristic ofthe infinite baffle is that the low frequency response capability isincreased with an increase in size of the enclosed space. For asatisfactory response at 50 cycles this requires an enclosed spaceequivalent to approximately ten times the cubed nominal diameter of theacoustic diaphragm. If the enclosed space is packed with fiber glass, asdisclosed in U.S. Pat. No. 2,775,309 to E.M. Villchur, an enclosed spaceequivalent to approximately two and one-half times the cubed nominaldiameter of the acoustic diaphragm will produce an equally satisfactoryresponse at 50 cycles, thus attaining a substantial reduction in size ofthe speaker enclosure. These devices are relatively low acousticimpedance devices because of the low impedance of large chambers.

The sound translating device disclosed in the Villchur patent may beregarded as utilizing an acoustic capacitance and an acousticalresistance couple or network, effective at low frequencies, in which thecapacitance acoustically charged and discharged from applied fluidcompressions and decompressions generated from the back of the relatedacoustic diaphragm is retarded by the acoustical resistance of a fiberglass packing therein effecting a time lag that may be consideredanalogous to certain R-C timing circuits used in certain electronicdevices. The nature of the fiber glass resistance, however, is such thatit is not effective as a means of further lowering the frequencyresponse by means of increasing its density beyond a certain optimum fora given volume of enclosure.

The quest is for more compact and better speakers.

Accordingly, the present invention provides very compact loudspeakers orother electro-acoustic translating devices having a satisfactory widefrequency range of response with an enclosure that may be of appreciablyless than one times the nominal diameter of the acoustic diaphragmcubed. The present invention, due to high pressures and small volumes,is herein regarded as a high impedance acoustic device, or sound cellbecause of the high acoustic impedance of the small chamber or chambers,and the high acoustic impedance of the duct.

it is one object of the present invention to provide an electro-acoustichigh acoustic impedance transducer, wherein a satisfactory wide range ofaudio response from a suitable operable acoustic diaphragm mounted in avery compact housing or enclosure is obtained, in which small airchambers functioning as air capacitances are interconnected through anextraordinary high acoustic resistance due to viscosity in an air ductconductor defined by contiguously opposing walls of great area ascompared with the cross-sectional area of the duct and in which thefirst capacitance in cooperation with the second of the air capacitancesor, in other words, the acoustic regeneration chamber acts as anacoustic resistancecapacitance network couple that reacts favorably tolow frequency cycles of compression and decompression that are impressedvia that first or acoustic pressure generation chamber from the back ofa related acoustic diaphragm.

The present invention is also directed to electro-acoustic transducersin general, of the completely enclosed or infinite baffle typecontaining a resurgent high pressure acoustic system in which a highacoustic resistance is coupled with relatively small acousticcapacitances for response to lower frequencies of the acoustic spectrumof the general type as disclosed in my copending US. Pat. applications,Ser. No. 629,623, filed Apr. 10, 1967, entitled "Electro-AcousticTransducer", and Ser. No. 619,658, filed March 1, 1967, now abandonedentitled Electro-Acoustic Transducer, and in my U.S. Pat. No. 3,324,966,granted June 13, 1967, entitled Electro-Acoustic Transducer, and No.3,317,000, granted May 2, 1967, entitled "Elcctro-Acoustic Transducer.

In the aforementioned there are disclosed enclosed acoustical systems inwhich the related acoustic diaphragm is predominantly controlled by theacoustic characteristics of the enclosed air or fluid in the housingaccording to the properties ofv a system of acoustic elements consistingof fluid compliance, resistance and mass contained in an arrangement ofchambers and connecting duct or ducts, which improves the fidelity ofthe reproduced sound corresponding to the variations ofelectrical input.

As disclosed therein, the principal feature is in the utilization of theresurgence of acoustic energy from a highly compressed fluid in a smallsecondary chamber controlled through a high fluid flow resistant ductback to the primary chamber or enclosure, in which the acoustic pressurevariations are generated on the back of the related acoustic diaphragm,thereby causing the fluid resurgence to react effectively on the back ofthe acoustic diaphragm increasing the efficiency of operation, that is,an acoustic resistance-compliance network designed to reproduce optimumacoustic response comparable to the electrical input.

In particular, the low frequency response capability of the infinitebaffie is increased, for a decrease in the acoustic capacitanceheretofore contained in the enclosure, for very compact loudspeakers orreceivers. This is achieved as indicated, with an operative acousticdiaphragm mounted in a small housing in which two very small airchambers, functioning as air capacitances, are interconnectecd by anextraordinarily high acoustic resistance in the form of a highresistance air duct conductor of substantial area relative to its widthbetween walls, which in cooperation with one of the two air capacitancesacts as an acoustic resistance-capacitance network couple, that reactsfavorably to low frequency cycles of compression and decompression thatare impressed via the first chamber from the back of a related acousticdiaphragm.

The specific embodiments disclosed in U.S. Pat. No. 3,317,000 consist ofan arrangement of contiguous walls forming an air conductor passagewhich is generally peripheral to the unit and axially disposed thereto,with the second or outer chamber located entirely rearwardly of theinner chamber.

The specific constructions disclosed in U.S. Pat. No.

3,324,966 consist of an arrangement of contiguous walls forming anannularly radial fluid medium passage disposed perpendicularly andradially to the unit; and wherein further, two annular chambers areformed about the magnetic structure of the unit, permitting compactloudspeaker systems, with the second chamber located entirely rearwardlyof the inner chamber. The copending patent applications generallydisclose an arrangement of wall means disposed in an enclosed space atthe back of the diaphragm, which wall means may also include thefluidtight enclosure, or be separate therefrom, and which wall meansforms contiguous radial walls defining a narrow radial or annular spacetherebetween, which narrow space communicates with the fluid medium inthe enclosed spaces, and with the second chamber substantially smallerthan the first chamber, and located entirely rearwardly andsubstantially radially inwardly relative to the inner chamber.

in all my patents and copending patent applications, the second orremote chamber is smaller or at least no larger than the inner chamberand located rearwardly relative thereto, and the inner or first chamberadjacent the diaphragm is located, at least in part, annularly about thesecond chamber, with'the latter being inwardly disposed in its entiretyrelative to the annular or surrounding part of the first chamber.

However, particularly, the present invention is directed towardconstructional and operational improvements in duct transducer devicesproviding still better performance and compactness and adaptability tovarious applications.

It is an object of the present invention to provide electroacoustictransducers having an inner chamber adjacent the diaphragm and an outerchamber communicating therewith through a high resistance duct, theouter chamber being posi' tioned rearwardly and/or at least in partannularly around the inner chamber and approximately equal to orsubstantially larger in volume than that of the inner chamber.

It is another object of the present invention to provide electro-acoustic transducers in accordance with the above mentionedobjective wherein the outer chamber is at least partly located annularlyabout the inner chamber for the purpose of a thin flat construction.

It is another object of the present invention to provideelectro-acoustic transducers in accordance with the above mentionedobjectives wherein the high resistance duct is axially arranged.

With these and other objects in view which will become apparent in thefollowing detailed description, the present invention will be clearlyunderstood in connection with the accompanying drawings, in which:

FIG. 1 is an axial section of an acoustic transducer designed inaccordance with the present invention; and

FIG. 2 is an axial section of another embodiment of an acoustictransducer in accordance with the present invention.

Referring now to the drawings and in particular to FIG. 1, theelectro-acoustic transducer of the present invention comprises aconventional dynamic loud speaker unit consisting of a conical acousticdiaphragm 1 suspended by a compliant peripheral flange 2 cemented to asupporting rim 3 of a basket 4 of a magnetic structure 5 actuated by avoice coil 6 which operates in a magnetic gap 7 in the magneticstructure 5, and

provided in a housing enclosure which includes a rear circular wall 9spaced axially and parallel to a front annular wall 8 and which alsoincludes an outer peripheral cylindrical wall 10 having circumferentialgrooves 10a and 10b in which the enclosure walls 8 and 9, respectively,are sealingly secured.

A toroidal solid member 11 having a substantially square cross section(although not limited thereto) is disposed coaxially about and sealinglyagainst the outer peripheral surface of the magnetic structure 5. Theouter and inner peripheral surfaces lla and 11b, respectively, of themember 11 are cylindrical and coaxially oriented relative the unit. Arelatively thin cylindrical wall baffle member 12 extends sealingly fromthe rim 3 of the basket 4 rearwardly short of the rear wall 9 and iscoaxially oriented relative the unit and spaced slightly apart andaround the outer peripheral surface 11a of the solid member 11.

The cylindrical member 12 thereby is positioned with its innercylindrical surface 12a coaxially aligned parallel and spaced from theouter cylindrical surface 11a of the member 11, forming therewith anaxial circumferential channel 13.

Members 11 and 12 divide the diaphragm enclosed housing into a first orinner chamber 14 adjacent the diaphragm, and a diaphragm chamber orsecond or remote outer chamber 15, both chambers in fluid communicationvia the axial channel or duct 13.

As clearly disclosed in FIG. 1 of the drawing, the outer chamber 15 isannular, surrounding entirely the inner chamber 14, and is bounded bythe enclosure s walls 8, 9 and 10 and by member 12 and the basket rim 3,whereas the inner chamber 14 is defined by the member 12, the basket rim3, the diaphragm l and the front surface 11c of the solid member 11, thelatter chamber being substantially triangular in cross section.

In accordance with the present invention, the inner chamber 14 ispreferably for wide range reproduction smaller in volume than that ofthe outer chamber 15.

In the embodiment illustrated in accordance with the present invention,the rim 3 of the basket 4 is formed in a plurality of preferably threesteps or angular rim portions 30, 3b and 3c, respectively. Against theinner angular rim portion 3a is disposed sealingly the front end of thecylindrical member 12. The compliant peripheral flange 2 of thediaphragm is cemented to the intermediate rim portion 3b of the basket4, and the outermost rim 3b of the basket 4 is sealingly secured to theinner portion of the frontenclosure wall 8. The rim 3 sealinglyseparates the chambers 14 and 15, and isolates the chamber 14 from theambient.

In accordance with the present invention, the front housing wall 8extends radially sufficiently outwardly to define the outer chamber 15to be relatively large in volume with respect to the inner chamber 14.The outer chamber 15 extends completely annularlyaround, as well asrearwardly, the inner chamber 14.

The solid member 11, being in a fluid-tight forced fit with respect tothe magnetic structure 5, insures that there is no fluid leakage betweenthe chambers 14 and 15 through the interface (surface 11b thereof.

Fiber glass packing 16 may be provided optionally within the chambers 14and/or 15 to optimize the density for suitable damping.

Spacing elements (not shown) may be used between the baffle member 12and the outer peripheral surface 11a of the solid member 11 to insurethe proper duct spacing. In accordance with the present invention, thereis thereby formed a back loading acoustic resurgence network assembly.constituting a pair of cylindrical surface cooperating members spacedcoaxially, and together with the housing enclosure rim 3 and thediaphragm 1, define the inner chamber 14, immediately adjacent andbehind the diaphragm, and the outer annular fluid resurgence chamber 15annularly disposed about chamber 14 about the member 12, both chamberscommunicating with each other via the axially circumferential duct 13between the members 11 and 12.

This permits a fluid medium behind the diaphragm l to pass only betweenthe chambers or cavities l4 and 15 through the axially disposedcircumferential duct 13.

The relative sizes of the chambers 14 and 15, respectively, are ofimportance to the function of the device, although the invention is notrestricted thereto. The acoustic resurgence network assembly of thepresent invention provides the secondary chamber 15 as a relatively lowcompression chamber, with the high resistance duct or conduit 13communicating the chamber 15 with the chamber 14.

With the structure of the present invention and the rear wall 9 disposedimmediately behind the rear end of the magnetic structure 5 and theouter chamber 15 being annularly provided thereabout, space requirementsare minimized especially the thickness of the unit.

In operation, the movements of the diaphragm 1 impelled by the energizedvoice coil 6, generate compressions and decompressions of the air in thechamber 14, which being of a small volume, causes plus and minuspressures to attain exceptionally high degrees above and below theatmosphere pressure, and thereby provides an air stiffness and controlover the movements of the diaphragm l, which predominates over themechanical stiffness of the compliances of the diaphragm l. The airstiffness thereby takes over much of the inertia of the moving parts.The small volume of chamber 14 relative chamber 15 provides furtherefficiencies.

The air duct or conduit 13 slowly releases the higher pressures in thechamber 14, which are generated increasingly with decrease of frequencyto chamber 15, where plus and minus pressures are regenerated again, butwith a time lag as compared with the pressures prevailing in the innerchamber 14 and reach a peak when the incoming air pressures through theair duct 13 become equal to the air pressures in the chamber 15. Theregenerated pressures in the chamber 15, which act as an acousticregenerating or reflection unit for low frequencies, become slowlyresurgent with the reversal of air flow adding the released powerresurgence through this air duct 13 to the back of the acousticdiaphragm 1 at a frequency compatible to that in the acousticresistance-capacitance network couple of the air duct 13 with thechamber 15. The relatively large size of the rear chamber with respectto the inner chamber 14 provides this slower resurgence than heretofore.

Because of the small housing desired for such a loudspeaker, theacoustic capacity of chambers 14 and 15, are comparatively minute andthe complementary acoustic resistance necessary to achieve a lowfrequency resurgence of 50 cycles or less is extraordinarily great. Thehigh acoustic resistance is achieved in accordance with the extremelyclose contiguous radial spacing, that is, in the order of a fewthousandths of an inch to one hundred thousandths, more or less,depending upon the size of the reproducer, between the cylindricalsurfaces 11a and 12a of the cooperating members 11 and 12, respectively,which define the air duct conductor 13, thus providing the means forattaining an extraordinarily high acoustic resistance in the great areaof the duct conductor 13. With this arrangement there is applied a highpressure by virtue of the small capacitanees, resistance controlledacoustic loading system, in which the acoustic resistance is very greatand is an effective retarding element, as well as of economicalconstruction, to the flow of air between the two relatively smallchambers 14 and 15. Such highly acoustic resistance is obtained by theannular air duct 13 of the viscosity effect of extensive surface area ascompared to the cross-sectional area of the duct. Accordingly, a highpressure resistance controlled regenerative acoustic loading system isachieved, acting on the back of the acoustic diaphragm, in which a highacoustic resistance is provided with the great viscosity residing in athin axially aligned air duct conductor formed intermediate the coaxialcylindrical contiguous surfaces lla and 12a of substantial length ascompared to the cross-sectional area of such air duct conductor.

The solid member 11 serves not only with its surface walls defining theinner surface of the air duct 13, and the rear surface 110 of the innerchamber 14, but also acts as a filler member filling an annular portionof the space in the housing enclosure space immediately behind the innerchamber and inwardly of the outer chamber 15, and may be readily,cheaply and inexpensively manufactured.

Referring now to the drawings and in particular to FIG. 2, anotherembodiment of the present invention is illustrated with like numeralsreferring to similar portions described already in connection withFIG. 1. In this embodiment of the loud speaker, an innermost cooperatingduct member 17 is provided, having an L-shaped cross section, and isannular in configuration, having an annular wall 18 and a cylindricalwall 19 perpendicular thereto. The member 17 is disposed sealingly influid tight engagement against the magnetic structure 5 at the innercircumference of the annular wall 18.

The cylindrical wall 19 of the member 17 is oriented coaxially relativethe unit and spaced coaxially inwardly relative baffle member 12. Theannular wall 17 defines the spacing between the outer peripheralcylindrical surface 19a of the cylindrical wall 19 and the innerperipheral surface 12a ofthe member 12. Spacers (not shown) may also beused.

The wall 17 is disposed at the front end of the magnetic structure 5with the wall 19 extending rearwardly therefrom at the outercircumferential edge of the wall 17, and terminates at rear end 1% shortof the rear housing wall 9'. Accordingly, there is formed a space orchamber portion or part 15b rearwardly of the annular wall 18 andbounded by the annular wall 18, the magnetic structure 5 and by andinwardly of the cylindrical wall 19 of the member 17, and which chamberportion 15b is in communication with the chamber portion 15a (similar tochamber 15 of FIG. 1 but smaller) defined between the housing walls 8, 9and 10, the chamber portions 15a and 15b constituting the second orouter chamber divided into two annular parts freely (without substantialresistance) in communication via rear communicating space 20 (betweenthe wall 9 and the rear edge 19b ofcylindrical wall 19).

The inner chamber 14 in the embodiment of FIG. 2 is similar to the innerchamber 14 of FIG. 1 in shape and size. The outer chamber 150, however.is substantially smaller than the chamber 15 of FIG. 1 and accordinglythe enclosure walls 8' and9 can be smaller in diameter, since chamberportion 1512 further constitutes the remainder of the outer chamber andserves to reduce the overall diameter of the entire unit.

The relative sizes of the inner chamber 14 and the outer chamber(constituting portions 15a and 15b), respectively, are of importance tothe functioning of the device. The volume of the chamber 14 is afraction of the volume of the outer chamber 15a and 15b. The acousticresurgent network assembly of the embodiment of FIG. 2 provides a highcompression chamber, namely chamber 14, and a high resistance conduit 13between the inner chamber 14 and the outer chamber (15a and 15!;together).

In operation the movements of the diaphragm 1 impelled by the energizedcoil 6 generate compressions and decompressions of air in the chamber 14which, being of small volume. cause plus and minus pressures to attainexceptionally high degrees above and below the atmosphere pressures and,thereby, provide an air stiffness and control over the movements of thediaphragm 1, which tends to predominate over the mechanical stiffness ofthe compliances in the diaphragm 1. The air stiffness thereby takes overmuch of the inertia of the moving parts. The air duct conductor 13slowly releases the higher pressures in the chamber 14, which aregenerated increasingly with decrease of frequency to the outer annularfluid resurgence chamber 15a and 15b, where plus and minus pressures areregenerated again, but with a time lag as compared with the pressuresprevailing in the first chamber 14, and reach a peak when the incomingair pressures through the air duct conductor 13 become equal to the airpressures in the outer chamber 15a and 15b. The regenerated pressures inthe outer chamber, which act as an acoustic regenerating or reflectingunit for low frequencies, become resurgent with the reversal of air flowadding the released power of resurgence through this air duct conductor13 to the back of the acoustic diaphragm 1 at a frequency compatible tothat in the acoustic resistance-capacitance couple of the air ductconductor 13 with the air chamber 15a and 15b.

The embodiment of FIG. 2 provides, in effect, a hollow portion or space(which was filled in FIG. 1 by the solid member 11) and thereby permitsreduction of the diameter of the entire unit while, at the same time,permitting the ratio of the combined outer chamber (15a and 15b), thechamber 15 being now a portion of the outer chamber, to be much greaterthan that of the inner chamber 14, providing the exceptional loudspeakerperformance in accordance with the present invention.

The high acoustic resistance is achieved in accordance with theextremely close contiguous axial spacing, that is, in the order of a fewthousandths of an inch, for extremely small units extending over therange of about one half of a thousandth, as might be for a minutehearing aid, to about one hundred thousandths of an inch, preferablyabout .06 inch, more or less, for a 4 inch diameter speaker unit,between the surfaces 12a and 19a, respectively, which define the airduct conductor 13, thus providing the means for attaining anextraordinarily high acoustic resistance in the great area of surfaceviscosity provided as compared to the minute cross-sectional area of theduct conductor 13.

Fiber glass packing 16 may be provided optionally in chambers 14 and/or15a and/or 15b to optimize the density for suitable damping.

With this arrangement there is applied a high pressure by virtue of thesmall capacities, resistance controlled acoustic loading system in whichthe acoustic resistance is very great and is an effective retardingelement, as well as of economical construction to the flow of airbetween the two relatively small chambers, in which the second chamberis substantially larger than the volume of the inner chamber. Such highacoustic resistance is obtained by the axial air duct 13 of theviscosity effcct of extensive surface area as compared to thecross-sectional area of the duct. Accordingly, a high pressureresistance controlled regenerative loading system is achieved acting onthe back of the acoustic diaphragm, in which a highly acousticresistance is provided in the great viscosity residing in a thin axiallyoriented air duct formed intermediate the axial cylindrical contiguoussurfaces of walls of substantial lengths as compared to thecross-sectional area of such air duct.

It is to be understood that other embodiments are possible, and that thescope of the present invention also may include various acoustic devicesemploying an operative acoustic diaphragm, the back of which acts on asystem of two small interconnected chambers in which the generated andregenerated acoustic resurgences from one to the other are controlled bya dominating high acoustic resistance in an air duct conductor definedby adjacent disposed wallsurfaces of an extensive area, as compared withthe cross-sectional area of the duct with the outer or second chamberbeing substantially greater in volume than that of the inner chamber. itis further understood that the term air is analogous with any fluidmedium, and the use of lighter or heavier than air fluid mediums in theresistance-capacitance system described, are within the scope of thepresent invention.

The present invention further provides in an electroacoustic transducer,a device consisting essentially of a Helmholtz resonator comprising acavity having the equivalent of a long tubular extension in the form ofa thin duct the outer terminus of which is open peripherally to anothercavity into which acoustic pressure variations are generated by anacoustic diaphragm fitted air tight into an opening therein, and meansto operate the diaphragm.

An electro-acoustic transducer comprises an operative acoustic diaphragmenclosed on one side with a small acoustic cavity resonant to upperaudio frequencies, the cavity peripherally relieved of the pressures ofthe lower frequencies by being in communication peripherally with theopen end of the equivalent of the tube in a Helmholtz resonator resonantto lower audio frequencies, and the combined resonances helping tocontrol the diaphragm to follow the variations of mixed frequencies ofelectrical input.

if the second cavity, the Helmholtz equivalent, is smaller than thefirst or input cavity, the upper the frequencies will predominate.

if the second, or Helmholtz cavity is larger than the first or inputcavity, the lower frequencies will predominate.

In either case the first or input cavity would cover the area of theacoustic diaphragm, but in depth would be only a fraction of thediametrical dimension of the diaphragm, that is the depth of the firstcavity would be preferably one-fourth of the diametrical dimension ofthe diaphragm or even less.

By virtue of its smallness, the first cavity is resonant to the higherfrequencies.

By virtue of the Helmholtz effect produced by the second cavity incooperation with the thin duct open at its terminous to the periphery ofthe first cavity, resonance to lower frequencies is provided. If thecavity of the Helmholtz resonator is empty, the resonant frequency willbe peaked. If the cavity is packed with fiber glass to optimum density,the lower frequency resonance peak is broadened which is desirable.

lclaim: 1. An electro-acoustic device comprising: a housing; an axialoperating means including an acoustic diaphragm and a magnetic structuregenerating acoustic pressure variations to a free fluid medium on oneside of said acoustic diaphragm and for generating correspondingacoustic pressure variations to a fluid medium enclosed in said housingon the other side of said acoustic diaphragm;

said housing on the other side of said acoustic diaphragm beingfiuidtight and defining an enclosed space;

wall means disposed within said enclosed space being two continguouswalls defining a narrow air duct intermediate said contiguous walls andifividing said enclosed space into a diaphragm chamber adjacent saidacoustic diaphragm and a second or outer chamber;

said narrow air duct in fluid communcation between said diaphragm andsecond chambers; and

said second chamber is at least in part annularly arranged about saiddiaphragm chamber.

2. The electro-acoustic device, as set forth in claim 1, wherein saidsecond chamber further extends rearwardly relative said diaphragmchamber.

3. The electro-acoustic device, as set forth in claim 2, wherein saidsecond chamber is substantially entirely arranged annularly with respectto said diaphragm chamber.

4. The electro-acoustic device, as set forth in claim 3, wherein saidwall means includes a solid toroidal member disposed coaxially andrearwardly of said diaphragm chamber, filling the space directlytherebehind, and positioned against said magnetic structure and definingon one surface thereof the rearmost surface of said diaphragm chamber.

5. The electro-acoustic device, as set forth in claim 4, wherein saidmember is substantially rectangular in cross section.

6. The electro-acoustic device, as set forth in claim 5, wherein saidwall means further includes a cylindrical regenerating means coaxiallyarranged about said member and spaced therefrom.

7. The electro-acoustic device, as set forth in claim 6, wherein saidregenerating means comprising a cylindrical baffie further extendingforwardly relative said member, separating thereat said diaphragmchamber from said second chamber.

8. The electro-acoustic device, as set forth in claim 1 wherein saidsecond chamber is in substantial part arranged annularly about saiddiaphragm chamber defining at least partly an outer part of said secondchamber, and said second chamber further is in substantial part locateddirectly behind said diaphragm chamber immediately about said magneticstructure defining an inner part of said second chamber.

9. The electro-acoustic device, as set forth in claim 8, wherein saidwall means include a member with an annular radially oriented walldisposed coaxially and rearwardly of said diaphragm chamber and againstand about said magnetic structure at the forwardmost portion thereof,thereby separating the rear of the diaphragm chamber from said innerpart of said second chamber directly therebehind.

10. The electro-acoustic device, as set forth in claim 9, wherein saidmember has an L-shaped cross section with an outermost cylindrical wallextending rearwardly from the outer circumferential edge of said annularradially oriented wall, and surrounding said inner part of said secondchamber, and said wall means further includes a cylindrical baffle,coaxially arranged about said cylindrical wall and spaced therefrom,defining therebetween said narrow air duct and separating thereaboutsaid outer part of said second chamber.

11. The electro-acoustic device, as set forth in claim 10, wherein saidcylindrical baffle further extends forwardly relative said cylindricalwall, separating thereat said diaphragm chamber from said outer part ofsaid second chamber, and said cylindrical baffle and said cylindricalwall spaced at their rear ends from the rear of said housing, therebycommunicating said inner part of said second chamber and said outer partthereof directly adjacent the rear end of said narrow air duct.

12. The electro-acoustic device, as set forth in claim 11, wherein saidouter part of said second chamber is of greater volume than that of saidinner part of said second chamber.

13. The electro-acoustic device, as set forth in claim 12, wherein saidhousing comprises a rear circular wall coaxially arranged relative, andspaced rearwardly behind near said magnetic structure, a front annularwall coaxially arranged about the edge of said acoustic diaphragm, andan outer cylin- -drical housing wall having front and rearcircumferential grooves on its inner surface adjacent its respectiveends, and said front annular wall and said rear circular wall disposedsealingly in said front and rear grooves, respectively.

. wherein said housing including an 14. The electro-acoustic device, asset forth in claim 1, wherein said wall means are arranged so that saidnarrow air duct is axially positioned.

IS.' The electro-acoustic device, as set forth in claim 14,

outer cylindrical housing wall, said wall means includes a cylindricalbaffle inwardly spaced coaxially relative said outer cylindrical housingwall, defining, at least in part, said second chamber therebetween andseparating said first chamber inwardly located relative thereto, anddefining said axially positioned narrow air duct communicating saiddiaphragm and second chambers.

16. The electro-acoustic device, as set forth in claim 15,

0 behind said diaphragm chamber, constituting a portion of said secondchamber.

